1. Field of the Invention
The present invention relates to a reference frequency generator, and more specifically to a digitally controlled crystal oscillator and a dual mode tuning method thereof.
2. Description of Related Art
In a radio communication system, a frequency synthesizer generates a channel frequency used by a receiver and a transmitter. The frequency synthesizer includes a Phase-Locked Loop (PLL) and generates a desired channel frequency using a predetermined reference frequency.
A reference frequency oscillator generates the reference frequency having an accuracy needed to stably synthesize the channel frequency and for a stable radio communication. The reference frequency oscillator includes a crystal resonator having a high Q value. The crystal resonator may have a frequency error due to various factors such as temperature variation, initial crystal offset, aging and the like.
A Voltage-Controlled Temperature Compensated Crystal Oscillator (hereinafter referred to as “VC-TCXO”) directly and indirectly compensates for the frequency error of the crystal resonator.
The VC-TCXO includes a voltage control oscillator for oscillating at a resonance frequency of the crystal resonator. The VC-TCXO controls the voltage control oscillator using an analog circuit configuration, and indirectly compensates for the frequency error of a crystal resonator. For example, the VC-TCXO uses a temperature compensation circuit having a thermistor, and controls a varactor diode capacitance of an LC voltage control oscillator so as to compensate for a frequency variation of the crystal resonator.
The VC-TCXO needs to be implemented as a different chip, separated from a transceiver chip. Such an implementation adds to the manufacturing costs and the area needed for mounting.
A Digitally Controlled Crystal Oscillator (hereinafter referred to as “DCXO”) has been used in products instead of the VC-TCXO.
The DCXO is the reference frequency oscillator that compensates for the frequency error of the crystal resonator and generates the reference frequency using both an analog circuit and a digital circuit.
The DCXO includes the circuits of the VC-TCXO except for the crystal resonator. The DCXO and the transceiver chip can be implemented as one chip. In comparison with the VC-TCXO, the DCXO is suitable for a wireless mobile communication terminal having size and weight limitations. The integrated chip has a reduced mounting area and may be manufactured at a reduced cost.
The DCXO can use a dual mode tuning technique so as to expand a tuning range and increase tuning speed. The dual mode tuning technique includes of both coarse mode tuning and fine mode tuning.
The coarse mode tuning is a process for compensating for a change of an offset frequency in each crystal resonator and a change of a load capacitance depending upon a manufacturing process change. The coarse mode tuning is performed by selecting a coarse mode signal composed of a digital control code inputted through a digital interface from an external source. The digital control code may be determined through calibration of a manufacturing process and stored in a table, or the like.
The fine mode tuning is a process for tuning a frequency more finely after the coarse mode tuning is completed. The fine mode tuning is also a process of compensating for the frequency error due to aging or temperature variation of the crystal resonator, thereby being minutely performed on a scale of sub-parts per million (ppm). The digitally controlled crystal oscillator for the fine mode tuning receives an Automatic Frequency Control (AFC) signal from the external source, for example, a modem chip.
FIG. 1 is a block diagram of a dual mode digitally controlled crystal oscillator.
Referring to FIG. 1, the dual mode tuning digitally controlled crystal oscillator 100 includes an off-chip crystal resonator 101, an on-chip oscillation circuit 111, a coarse mode tuning unit 112, and a fine mode tuning unit 113.
The oscillation circuit 111 uses an LC oscillator that generates an oscillation frequency using LC resonance.
The coarse mode tuning unit 112 performs a coarse mode tuning for the oscillation circuit 111. The coarse mode tuning unit 112 receives a coarse mode tuning signal (CO_TUNE) from the external source and controls capacitance of an oscillator tank by controlling a capacitor bank included in the oscillator 111 in response to the received tuning signal (CO_TUNE).
The fine mode tuning unit 113 performs a fine mode tuning for the oscillation circuit 111. The fine mode tuning unit 113 receives a fine mode tuning signal (FI_TUNE) from the external source, and oscillates a reference frequency (FREF) by controlling the oscillation circuit 111 in response to the received tuning signal (FI_TUNE). The fine mode tuning signal (FI_TUNE) may be an AFC signal received from the modem chip.
The coarse mode tuning signal (CO_TUNE) and the fine mode tuning signal (FI_TUNE) may be composed of a digital or an analog control signal. In case that the coarse mode tuning signal (CO_TUNE) and the fine mode tuning signal (FI_TUNE) are composed of a digital control code, the coarse mode tuning unit 112 and the fine mode tuning unit 113 may operate by receiving the digital control code and by controlling the capacitor bank included in the oscillation circuit 111. Alternatively, in case that the coarse mode tuning signal (CO_TUNE) and the fine mode tuning signal (FI_TUNE) are composed of an analog control signal, the coarse mode tuning unit 112 and the fine mode tuning unit 113 may operate, for example, by receiving a control voltage and by changing the capacitance of the varactor diode.
The coarse mode tuning and the fine mode tuning respectively have a tuning range and a tuning resolution. It is desirable for a fine mode tuning range to cover only a minimum unit interval of the coarse mode tuning range. In an actual implementation, the fine mode tuning range is typically designed to cover a range about 20 times as large as the minimum unit interval of the coarse mode tuning range, because the fine mode tuning has to compensate for a frequency change due to aging or temperature variation of the crystal resonator. The fine mode tuning simultaneously performs the frequency tuning because the coarse mode tuning signal is maintained without change after performing the coarse mode tuning. Additionally, since the coarse mode tuning signal for the coarse mode tuning has the same value for all samples during a calibration operation, the fine mode tuning should compensate for a difference of the crystal resonator between samples and a change of the load capacitance. Accordingly, a fine mode tuning range needs to be greater than the minimum unit interval of the course mode tuning range.
Likewise, the increased width of the fine mode tuning range increases the size of the varactor diode needed for the fine mode tuning of an analog manner and the capacitor bank needed for the fine mode tuning of a digital manner. Accordingly, the load capacitance increases, and costs in design and difficulties of a manufacturing process simultaneously increase.
Therefore, a need exists for a digitally controlled crystal oscillator and a dual mode tuning method thereof.